Magnetic recording media are widely used in various applications, particularly in the computer industry. A portion of a conventional recording medium 1 utilized in disk form in computer-related applications is schematically depicted in FIG. 1 and comprises a non-magnetic substrate 10, typically of metal, e.g., an aluminum-magnesium (Al—Mg) alloy, having sequentially deposited thereon a plating layer 11, such as of amorphous nickel-phosphorus (NiP); a seed layer 12A of an amorphous material,. e.g., NiAl; a polycrystalline underlayer 12B, typically of chromium (Cr) or a Cr-based alloy; a magnetic layer 13, e.g., of a cobalt (Co)-based alloy; a protective overcoat layer 14, typically containing carbon (C), e.g., diamond-like carbon (“DLC”); and a lubricant topcoat layer 15, typically of a perfluoropolyether compound, applied by dipping, spraying, etc.
In operation of medium 1, the magnetic layer 13 can be locally magnetized by a write transducer or write head, to record and store data/information. The write transducer creates a highly concentrated magnetic field which alternates direction based on the bits of information being stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the recording medium layer 13, then the grains of the polycrystalline medium at that location are magnetized. The grains retain their magnetization after the magnetic field produced by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The pattern of magnetization of the recording medium can subsequently produce an electrical response in a read transducer, allowing the stored medium to be read.
Thin film magnetic recording media are conventionally employed in disk form for use with disk drives for storing large amounts of data in magnetizable form. Typically, one or more disks are rotated on a central axis in combination with data transducer heads. In operation, a typical contact start/stop (“CSS”) method commences when the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by the air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates, such that the head can be freely moved in both the circumferential and radial directions, allowing data to be recorded on and retrieved from the disk at a desired position. Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from the static position, and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic sequence consisting of stopping, sliding against the surface of the disk, floating in air, sliding against the surface of the disk, and stopping.
It is considered desirable during reading and recording operations, and for obtainment of high areal recording densities, to maintain the transducer head(s) as close to the associated recording surface(s) as is possible, i.e., to minimize the “flying height” of the head(s). Thus a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head, thereby permitting the head and the disk surface to be positioned in close proximity, with an attendant increase in predictability and consistent behavior of the air bearing supporting the head during motion.
Disk drives typically comprise a magnetic head assembly mounted on the end of a support or actuator arm which positions the head radially over the disk surface. If the actuator arm is held stationary, the magnetic head assembly will pass over a circular path on the disk surface known as a track, and information can be read from or written to that track. Each concentric track has a unique radius, and reading and writing information from or to a specific track requires the magnetic head to be located above the track. By moving the actuator arm, the magnetic head assembly is moved radially over the disk surface between tracks.
The disk drive must be able to differentiate between tracks on the disk and to center the magnetic head over any particular track. Most disk drives use embedded “servo patterns” of magnetically recorded information on the disk. The servo patterns are read by the magnetic head assembly to inform the disk drive of the track location. Tracks typically include both data sectors and servo patterns. Each data sector contains a header followed by a data section. The header may include synchronization information to synchronize various timers in the disk drive to the speed of disk rotation, while the data section is used for recording data. Typical servo patterns are described in, for example, U.S. Pat. No. 6,086,961, the disclosure of which is incorporated herein by reference.
Servo patterns are usually written on the disk during manufacture of the disk drive, after the drive is assembled and operational. The servo pattern information, and particularly the track spacing and centering information, needs to be located very precisely on the disk surface. However, at the time the servo patterns are written, there are no reference locations on the disk surface which can be perceived by the disk drive. Accordingly, a highly specialized device known as a “servo-writer” is used during writing of the servo-patterns. Largely because of the locational precision needed, servo-writers are expensive, and servo-writing is a time-consuming process.
One approach (i.e., “PERM” disks, manufactured by Sony Corp.) to avoid traditional servo-writing has been to injection mold or stamp servo patterns on a polymer-based substrate disk. A constant thickness layer of magnetic recording material is then applied over the entire disk surface, including the depressions and protrusions of the servo patterns. After all of the constituent layers of the medium have been applied to the disk, a magnetic bias is recorded on the servo patterns. For example, a first magnetic field may magnetically initialize the entire disk at a one setting. Then a second magnetic field, located at the surface of the disk and e.g., provided by the magnetic head of the disk drive, is used to magnetize the protruding portions of the servo patterns relative to the depressions. Because the protrusions are closer than the depressions to the magnetic initialization, the magnetization carried by the protrusions may be different than the magnetization carried by the depressions. When read, the resulting disk servo patterns show magnetic transitions between the depressions and the protrusions.
Meanwhile, the continuing trend toward manufacture of very high areal density magnetic recording media at reduced cost provides impetus for the development of lower cost materials, e.g., polymers, glass, ceramics, and glass-ceramics composites as replacements for the conventional Al alloy-based substrates for magnetic disk media. However, poor mechanical and tribological performance, track mis-registration (“TMR”), and poor flyability have been particularly problematic in the case of polymer-based substrates fabricated as to essentially copy or mimic conventional hard disk design features and criteria. On the other hand, glass, ceramic, or glass-ceramic materials are attractive candidates for use as substrates for very high areal density disk recording media because of the requirements for high performance of the anisotropic thin film media and high modulus of the substrate. However, the extreme difficulties encountered with grinding and lapping of glass, ceramic, and glass-ceramic composite materials have limited their use to only higher cost applications such as mobile disk drives for “notebook”-type computers.
Sub-micron flyability (e.g., <0.5μ inch) of the recording transducer or head over a patterned media surface is a stringent requirement for obtainment of very high areal density recording media. However, attempts to achieve the requisite surface topography on glass, ceramic, or glass-ceramic composite substrates have been unsuccessful due to their extreme hardness (e.g., glass substrates have a Knoop hardness greater than about 760 kg/mm2 compared with about 550 kg/mm2 for Al alloy substrates with NiP plating layers). In addition, the low flowability and extreme hardness of these substrate materials effectively precludes formation of servo patterns in the surfaces thereof by injection molding or stamping, as has been performed with polymer-based substrates.
A recently developed approach for forming servo patterns in hard-surfaced, high modulus alternative substrate materials, such as the glass, ceramic, and glass-ceramic materials described above, is based upon the discovery that the surfaces of such materials may be modified, i.e., reduced in hardness, so as to facilitate formation of servo patterns therein, as by a simple and conveniently performed embossing process. According to this methodology, modification (i.e., reduction) of surface hardness of high modulus substrates for use in the manufacture of thin film magnetic recording media is obtained by first forming a relatively soft coating layer on the substrate surface, embossing the desired servo pattern in the exposed upper surface of the relatively soft coating layer, and then converting the relatively soft layer to a relatively hard layer while retaining the embossed servo pattern therein. The thus-formed substrate with embossed servo pattern in the exposed surface thereof is then subjected to thin film deposition thereon for forming the layer stack constituting the magnetic recording medium. Thus, the method advantageously provides servo-patterned magnetic recording media without requiring servo-writing subsequent to media fabrication.
More specifically, according to the above methodology, a relatively soft layer of a sol-gel is initially formed on the surface of the high modulus glass, ceramic, or glass-ceramic composite substrate, e.g., in disk form. By way of illustration, but not limitation, a sol-gel layer having a thickness of from about 0.2 to about 1 μm may be formed on the substrate surface by any convenient technique, e.g., spin coating of a solution of the sol-gel. A suitable sol-gel solution for use according to the invention may be prepared by mixing an alkoxide, e.g., a silicon alkoxide such as tetraethoxysilane (“TEOS”) or tetramethoxysilane (“TMOS”), water, and nitric acid at molar ratios of TEOS or TMOS/H2O/HNO3 of ¼–30/>0.05. The nitric acid acts as a catalyst for conversion of the TEOS or TMOS to a SiO2 sol according to the following reaction (1), illustratively shown for TEOS:nSi(OC2H5)4+2nH2O→nSiO2+4nC2H5OH  (1)with ethanol (C2H5OH) being produced as a reaction product in solution. After completion of reaction, butanol (C4H9OH) is added to the solution as a drying retardation agent at molar ratios of TEOS/H2O/HNO3/C4H9OH of e.g., 1/5/0.05/>4. Such solution, when applied to the substrate surface as by spin coating, forms a very smooth film with a minimum amount of surface microwaves. The spin coating process effects removal, e.g., as by centrifugation and evaporation, of a portion of the solvents from the initially applied solution. The resultant partially dried film or layer is glass-like, principally comprised of silica (SiO2) molecular clusters together with the various solvents (H2O, C2H5OH, C4H9OH), and adheres well to the substrate surface. The sol-gel film or layer is of a porous structure with the solvents saturated in the micropores thereof.
The as-deposited, relatively soft sol-gel film or layer applied to the hard-surfaced substrate is then subjected to an embossing process for forming a servo pattern in the surface thereof, comprising a patterned plurality of depressions and protrusions, e.g., by utilizing a stamper having a negative image of the desired servo pattern or an equivalently performing device. The exposed, upper surface of the relatively soft sol-gel layer may also be subjected to mechanical texturing (after drying in air but prior to sintering), e.g., as by a standard NiP texturing process utilizing an abrasive size of about 0.25 μm, in order to enable formation of oriented media critical for achieving high areal density recording.
Subsequent to servo pattern formation (and mechanical texturing, if desired) of the as-deposited, partially dried, relatively soft sol-gel film or layer, a sintering process is performed at an elevated temperature of from about 300 to above about 1000° C. (depending upon the withstand temperature of the substrate material, i.e., which temperature is higher for ceramic-based substrates than for glass-based substrates) at e.g., a ramping rate from about 0.5 to about 10° C./min. and a dwell time of about 2 hrs., to evaporate the solvents so as to effect at least partial collapse of the micro-pores, with resultant densification of the sol-gel film or layer into a substantially fully densified glass layer having a density and hardness approaching that of typical silica glass (<1.5 g/cm3), or into a partially densified “glass-like” layer. The embossed servo pattern (and mechanical texturing) formed in the exposed upper surface of the relatively soft sol-gel layer is preserved in the corresponding exposed upper surface of the sintered glass or glass-like layer.
Formation of thin film magnetic media on the thus-formed glass-coated, servo patterned/mechanically textured substrates is accomplished utilizing conventional thin film deposition techniques, e.g., sputtering, for forming the layer stack comprising a seed layer, polycrystalline underlayer, magnetic layer, and protective overcoat layer.
The above-described sol-gel-based process combines the advantages of low-cost processing with the superior optical, mechanical, and chemical properties of silica glass (SiO2) for fabricating high performance, servo-patterned magnetic recording media, and thus is an attractive candidate for future disk-based magnetic recording media and systems. In order to achieve high areal density magnetic recording utilizing such alternative substrate materials with embossed servo patterns formed in spin-coated sol-gel layers and with read/write transducers operating at very low flying heights, extremely precise (or faithful) replication of the servo master pattern on the imprinting surface of the stamper is required. However, the requisite degree or precision of servo pattern replication is generally not realized when a stamper is utilized to imprint/emboss the pattern in the surface of a sol-gel coated disk substrate, even when the imprinting/embossing is performed under a high pressure. It is believed that the poor replication quality arises from difficulty in compressing the partially dried sol-gel layer to the extent necessary for complete, i.e., faithful, replication of the topographical features, e.g., depressions and protrusions, in the imprinting surface of the stamper. For example, the depths of depressions and the wall angles of the features of servo patterns formed by pressure-induced embossing/imprinting of sol-gel coated disk substrates generally do not replicate the depths and sharpness of the corresponding features of the imprinting surface of the stamper. In particular, desired wall angles >450° and flat lands between adjacent grooves are generally not obtained.
In view of the above, there exists a need for improved methodology and means for providing disk substrates for magnetic recording media, which substrates are constituted of high modulus, very hard materials such as of glass, ceramic, or glass-ceramic, with at least one major recording surface having requisite topography for enabling operation with flying head read/write transducers/heads operating at very low flying heights and with very precisely contoured patterns, e.g., servo patterns, provided therein. More specifically, there exists a need for an improved method for rapidly and cost effectively forming patterned sol-gel films or layers on substrate surfaces, e.g., servo patterned films or layers, which patterns faithfully replicate master patterns. In addition, there exists a need for an improved, high areal density magnetic recording medium including a high hardness, high modulus substrate having a servo pattern integrally formed therewith in the surface of a densified, glass-like sol-gel layer, and capable of operation with read/write transducers operating at sub-micron flying heights.
The present invention addresses and solves problems and difficulties attendant upon the formation of faithfully replicated patterns, e.g., servo patterns, in the surfaces of sol-gel films on the surfaces of very hard materials, e.g., of glass, ceramic, or glass-ceramic, utilized as substrates in the manufacture of very high areal density magnetic recording media, while maintaining full capability with substantially all aspects of conventional automated manufacturing technology for the fabrication of thin-film magnetic media. Further, the methodology and means afforded by the present invention enjoy diverse utility in the manufacture of various other devices and media requiring formation of surfaces with precisely replicated formed therein.